1. Field of the Invention
An electromagnetic speedometer for a boat having hull containing an opening includes an electromagnetic coil supported by the hull for establishing an electromagnetic field in the water adjacent the hull opening. A plurality of electrodes are supported by an arrangement that both closes the hull opening and supports the electrodes in engagement with the water adjacent the hull opening. Preferably, the electrode support arrangement includes guide tubes supporting the electrodes for removal relative to the boat hull, thereby to permit cleaning of the electrodes. The guide tubes are closed by valves when the electrodes are removed from the assembly.
2. Description of Related Art
Electromagnetic flowmeters for measuring the flow of fluid in pipes are well known in the patented prior art, as shown by the inventor's Marsh U.S. Pat. No. 6,598,487, and the patent to Gardner U.S. Pat. No. 4,079,626.
In the boating industry, most pleasure craft utilize some sort of speed indicating device to determine the boat's speed relative to the water. Typical of such devices are paddle wheels (impellers), pitot tubes, ultrasonic sensors and electromagnetic sensors. Typical of such electromagnetic speed sensors and ultrasonic speed sensors that can utilize standard thru-hole fittings, such as the “Electromagnetic Log Sensor” manufactured by nke Marine Electronics, of Hennebont, France, and the “ALIZE Electromagnetic Speed Log” manufactured by AmeSys International, of Aux-en-Provence, France.
Paddle wheels and pitot tubes are prone to repeated fouling by marine growth as well as by debris in the water; whereas, ultrasonic sensors and EM sensors are less prone to foul. However, most all speed sensors are designed so that they can be removed from the boat and cleaned, even when the boat is in the water.
To facilitate the removal process, manufacturers typically supply the boat owner with a “sea valve” or a special thru-hull penetration that allows the boat owner to insert various sensors that must make contact with the water to properly function. Typically these sensors include “O-rings” that prevent water from leaking into the boat through this hull penetration.
Typically, the thru-hull penetration is approximately 1.5 inches in diameter creating a hole of approximately 1.75 square inches. FIG. 1 shows such a device. FIG. 2 shows the size and shape of a typical sensor along with its “O-rings”. When a boat owner is removing the sensor for periodic cleaning, he will typically first remove the actual sensor and then immediately install in the open hole a “dummy” sensor or “blank” to prevent the rapid inrush of water into the boat. If done properly, less than a gallon of water will flow into the bilge of the boat. Many users, however, are fearful of performing this procedure while their boat is in the water and will wait until the boat is pulled from the water to perform this operation. Some manufacturers of thru-hull penetrators have designed special back-flow flaps that attempt to reduce the inflow during maintenance. However, concerns still exist about the removal of a sensor while the boat is in the water.
A second problem associated with most existing boat speedometers is that the speed measurement is made very close to the boat's hull and in such a small volume of water that the speed measurement can be adversely effected by the boat's hull especially at high speeds and on larger boats. To partially compensate for effects of the boundry layer on the accuracy of the speed measurement users typically perform “at sea” calibrations over a range of speeds and establish corrections in the reported Speeds of the speedometer.
In the inventor's prior Marsh U.S. Pat. No. 6,598,487, a magnetic flowmeter is disclosed for measuring the flow of fluid in a conduit, including a separable electrode assembly in which a plurality of linear electrodes extend through self-sealing glands. This prior teaching differs from the measurement of the speed of a boat, since in the electromagnetic speedometer of the present invention, the sensor magnet and electrode means are specifically designed for mounting in an opening contained in the hull of the boat.